1. Field of Invention
This invention relates to a vortex flow metering apparatus to measure the velocity or quantity of a fluid by utilizing Karman's vortex.
2. Description of the Prior Art
It is well known that inserting an object into a fluid causes vortexes to arise alternately and regularly from both rear sides of the object in a flow downstream. The vortex flow is called Karman's vortex flow, and is a well known phenomenon. The number of vortexes (vortex generation frequency) generated in a unit time is proportional to the velocity of the fluid. A number of prior art devices employe this principle to measure the velocity, for example, of a fluid flow. For example, U.S. Pat. No. 4,248,098, discloses a vortex flow metering apparatus which operates to measure the velocity or quantity of a fluid by a method wherein a vortex generator is arranged in a line to introduce a measuring fluid, and a stress change according to a dynamic lift change resulting from the vortex generation is detected on a single piezoelectric sensor provided on the vortex generator (or receiver), and then subjected to a signal transformation. However, disadvantageously, this type of prior art vortex flow metering apparatus may be adversely affected or influenced, for example, by disturbance vibration, such as piping vibration caused by operation of the pump.
The disturbance vibration may touch off vibration of the vortex generator (or receiver), thereby resulting in vibration of the measuring equipment, including a converter circuit which is usually mounted on the line. When the vortex generator (or receiver) vibrates, a bending moment according to distribution of mass works on the vortex generator (or receiver). When the equipment vibrates, a strain arises on the line, and that strain will produce a bending moment on the vortex generator (or receiver). As a result, there is detected on the single piezoelectric sensor a phenomenon wherein one noise component produced by the bending moment due to the vibration of the vortex generator (or receiver) and another noise component produced by the bending moment due to the line strain, overlap with a signal component produced by the bending moment due to dynamic lift of the vortex. Thus, the conventional prior art vortex flow metering apparatus which uses only one piezoelectric sensor is disadvantageously subject to noise influence due to the disturbance vibration. Consequently, the signal to noise ratio (S/N) deteriorates, especially at low velocities. Thus, there is a deficiency in the prior art, and a great need exists for a vortex flow metering apparatus which can effectively eliminate the noise due to disturbance vibrations.